TIPP toppiest ever

I've been calling them "Undecideds" but I notice that TIPP lists them as "Not Sure." I thought "undecided" was the traditional category for someone who wasn't committed one way or the other but maybe I'm wrong. I'm not smart enough to dig into "cross-tabs" and all that other stuff poll-nuts do; hell, I don't even look at the questions. But I wonder if there is a difference among polls or the particluar questions asked that would account for the large number of "Not Sures" in the TIPP poll compared to the smaller number of "Undecideds" in other polls.